1. Field of The Invention
This invention relates to a shaped article of a tetrafluoroethylene polymer. More particularly, the present invention is concerned with a shaped article of a tetrafluoroethylene polymer of ultrahigh molecular weight which article has specific values with respect to a specific gravity, an orientation release stress, a shrinking starting temperature, an average maximum expansion ratio and a least maximum shrinkage ratio. The shaped article has excellent creep resistant and gas barrier properties, and has excellent dimensional stability and impact strength. Accordingly, it can be advantageously used as a sealant such as a gasket and a packing, as a lining material, a bearing pad, a sliding pad, sliding parts such as a bushing, a bearing and a piston ring, or the like. Especially, advantageous application of the present shaped article is found in packing, gaskets and bearings for automobile mechanical parts which must have high dimensional stability and impact strength at high temperatures.
2. Discussion of Related Art
Polytetrafluoroethylene is now an important engineering material due to its various excellent properties. In industry, polytetrafluoroethylene is now widely used as a sealant material such as a gasket and a packing, a lining material or the like, since the polymer is excellent in its resistance to chemicals, heat, and low temperatures, and has low frictional properties, non-staining properties, tack-free properties, electrical insulating properties and the like.
The melt viscosity of polytetrafluoroethylene is extremely high as compared with those of ordinary thermoplastic resins such as polystyrene, polyethylene, polyamide, acrylic resin and the like. Polytetrafluoroethylene has a melt viscosity as high as about 10.sup.11 poises even at a temperature of about 380.degree. C. which is higher than its melting temperature (327.degree. C.), whereas the melt viscosities of the ordinary thermoplastic resins are only about 10.sup.3 to 10.sup.4 poises at their melting temperatures. Due to such extremely high melt viscosity, polytetrafluoroethylene cannot be molded by the customary molding techniques such as melt extrusion, injection molding and the like which are generally employed for the molding of these ordinary thermoplastic resins. The methods which are useful for molding polytetrafluoroethylene are limited to special methods, for example, those called compression molding, ram extrusion molding and paste extrusion molding, in which a polytetrafluoroethylene powder is compressed under an appropriate pressure to obtain a molded article and the molded article is sintered at a temperature which is higher than the melting temperature thereof, followed by cooling. With respect to these methods, reference may be made, for example, to pages 53 to 54 of Satokawa et al "Fusso Jushi (Fluoro Resins)" published by the Nikkan Kogyo Shimbun Ltd., Tokyo, Japan, in April, 1976.
Further, it is noted that the temperature up to which polytetrafluoroethylene can be used when no load is applied thereto is 260.degree. C. However, in many applications, polytetrafluoroethylene is used under load, and the critical temperature up to which an article of polytetrafluoroethylene can be stably used is far lower than 260.degree. C., although it varies depending on the structure, configuration and field of application of the article. This is largely because polytetrafluoroethylene has an inherent creeping property under load, especially at high temperatures.
In the field of gaskets, primarily, two measures have been taken to deal with the problem of polytetrafluoroethylene creeping. One is to incorporate a filler material such as glass fiber, graphite, carbon fiber and zirconium oxide into the polytetrafluoroethylene. The other is to combine polytetrafluoroethylene or a filler material-incorporated polytetrafluoroethylene as mentioned above with a material having less of a tendency to creep, as seen in jacketed and spiral wound gaskets. However, the gaskets comprised of polytetrafluoroethylene and a filler material are not satisfactory since the gaskets are still insufficient in creep resistant properties at high temperatures, and since some types of filler materials cause the gaskets to have a decreased resistance to chemicals. On the other hand, the jacketed and spiral wound gaskets are disadvantageous in that for these, a high clamping pressure is needed and that they cannot be formed into complex forms. Therefore, there is still a strong demand in the art for polytetrafluoroethylene-based gaskets which have excellent creep resistant properties and are free from the above-mentioned disadvantages. With respect to the sealants as well, such as valve sealants, e.g. ball valve seats and gate valve seats, and dynamic sealants, e.g. gland packings, U packings and V packings, for which polytetrafluoroethylene is also advantageously used when they are required to have anticorrosive properties, improvement of the compression creep resistance of the article is also strongly desired in the art for the reasons as mentioned above with respect to the gaskets.
Monoaxially oriented tetrafluoroethylene polymer sheets as obtained by rolling the polymer under pressure exhibit an improved compression creep resistance as compared to that of non-oriented tetrafluoroethylene polymer sheets. However, for example, when a compressive force is applied to a sheet-form article of a monoaxially oriented tetrafluoroethylene polymer at a high temperature, e.g. 100.degree. C. or more, the article is shrunk in the counter-stretching direction, and is expanded in a direction perpendicular to the counter-stretching direction in a plane along the surface of the sheet. As a result, a substantial dimensional change occurs in the article. On the other hand, however, if a tetrafluoroethylene polymer is stretched without applying any pressure thereto, the resultant article is porous so that it is not suitable for the usages intended in the present invention.
The foregoing measure for dealing with the creeping property of polytetrafluoroethylene are not satisfactory since all of them have the drawbacks as mentioned above. Recently, proposals have been made to provide more satisfactory measures for dealing with the drawbacks of polytetrafluoroethylene articles. See U.S. Pat. No. 4,388,259 and U.S.S.R. Pat. No. 405,732. Specifically, to improve the creep resistant properties of a polytetrafluoroethylene gasket, it has been proposed, as disclosed in U.S. Pat. No. 4,388,259, to manufacture a gasket by a method comprising (a) heating a fluorocarbon polymer sheet of predetermined thickness to a temperature within the range at which the fluorocarbon polymer enters the gel state; (b) compressing said heated sheet to a thickness less than the above-mentioned predetermined thickness; (c) cooling the sheet in its compressed state; and (d) cutting the sheet, in its compressed, cooled state, into a gasket. However, the gaskets as manufactured according to the method of the patent do not have desirable creep resistant properties. On the other hand, with respect to U.S.S.R. Pat. No. 405,732, in one of the two Examples described in the specification thereof there is disclosed a rolling of polytetrafluoroethylene to attain a high orientation of the polymer. Rolling is generally employed to produce a monoaxially oriented continuous sheet. In the roll orientation, it is necessary to conduct rolling at elevated temperatures while applying the tensile force in the lengthwise direction of the sheet and to release the tensile force after cooling the sheet. Rolling is not suitable for effecting biaxial or multiaxial orientation of the polymer molecules. The other Example of the U.S.S.R. patent discloses compression of a preformed sheet of polytetrafluoroethylene in a press mold corresponding in form and dimension to the ultimate packing so as to cause the ratio of the thickness of the preformed sheet to the thickness of the ultimate packing to be 1.2 or more, which ratio may be regarded as a draw ratio. However, according to this method, this ratio is at most about 2.0 and a shaped article of a highly oriented polytetrafluoroethylene such as that having a draw ratio greater than about 2 cannot be obtained. The products obtained in the U.S.S.R. patent do not have desirable creep resistant properties.
Polytetrafluoroethylene is also important as an anticorrosive lining material. However, it has a drawback in that it tends to be accompanied by a blistering phenomenon, especially when it is employed in a pipe through which a gas-phase fluid is passed. The blistering is fatal from a viewpoint of the purpose of the lining. Hence, improvement of the blistering resistant properties of the poly tetrafluoroethylene article is strongly desired in the art. The blistering resistant properties of the polytetrafluoroethylene article can be represented by the gas barrier properties of the article. That is, a polytetrafluoroethylene article having improved gas barrier properties exhibits improved blistering resistant properties, and a polytetrafluoroethylene article having poor gas barrier properties exhibits poor blistering resistant properties.
Also, there are instances where tetrafluoroethylene polymer gaskets and linings suffer serious cracks and damages due to their poor impact strength. Accordingly, there is a strong demand in the art for a tetrafluoroethylene polymer article having desirable impact strength.